Abnormality diagnosis apparatus for exhaust gas purification apparatus

Abstract

An abnormality diagnosis apparatus for an exhaust gas purification apparatus including a selective catalytic reduction NOx catalyst (SCR catalyst) and a sensor which detects NOx and ammonia in exhaust gas flowing out from the NOx catalyst and at which NOx and ammonia react with each other. The abnormality diagnosis apparatus calculates, based on concentrations of NOx and ammonia which flow out from the SCR catalyst, an amount of decline in output of the sensor due to a reaction between NOx and ammonia at the sensor, and performs an abnormality diagnosis of the SCR catalyst based on the calculated amount of decline in output of the sensor.

The invention claimed is: 
     
       1. An abnormality diagnosis apparatus for an exhaust gas purification apparatus of an internal combustion engine, the exhaust gas purification apparatus including:
 a selective catalytic reduction NOx catalyst which is provided in an exhaust passage of the internal combustion engine and which uses ammonia as a reducing agent; 
 a supplying unit which supplies ammonia or an ammonia precursor into exhaust gas that flows into the selective catalytic reduction NOx catalyst on an upstream side of the selective catalytic reduction NOx catalyst; and 
 a sensor which is provided on a downstream side of the selective catalytic reduction NOx catalyst, which detects NOx and ammonia in exhaust gas flowing out from the selective catalytic reduction NOx catalyst, and at which NOx and ammonia react with each other, 
 the abnormality diagnosis apparatus for the exhaust gas purification apparatus comprising: 
 a NOx concentration estimating unit configured to estimate a NOx concentration in the exhaust gas flowing out from the selective catalytic reduction NOx catalyst; 
 an ammonia concentration estimating unit configured to estimate an ammonia concentration in the exhaust gas flowing out from the selective catalytic reduction NOx catalyst; 
 a calculating unit configured to calculate an amount of decline in output of the sensor due to a reaction between NOx and ammonia at the sensor, based on the NOx concentration estimated by the NOx concentration estimating unit and the ammonia concentration estimated by the ammonia concentration estimating unit; 
 a diagnosing unit configured to perform a diagnosis of the selective catalytic reduction NOx catalyst based on an output value of the sensor; and 
 a diagnosis control unit configured to control a diagnosis by the diagnosing unit based on the amount of decline in output of the sensor as calculated by the calculating unit. 
 
     
     
       2. The abnormality diagnosis apparatus for the exhaust gas purification apparatus according to  claim 1 , wherein
 the diagnosis control unit is configured to: 
 make the diagnosing unit perform an abnormality diagnosis of the selective catalytic reduction NOx catalyst when an amount of rise of a NOx purification rate calculated based on the amount of decline in output of the sensor as calculated by the calculating unit is equal to or smaller than a prescribed amount; and 
 make the diagnosing unit not perform an abnormality diagnosis of the selective catalytic reduction NOx catalyst when the amount of rise of the NOx purification rate calculated based on the amount of decline in output of the sensor as calculated by the calculating unit is larger than the prescribed amount. 
 
     
     
       3. The abnormality diagnosis apparatus for the exhaust gas purification apparatus according to  claim 1 , wherein
 the diagnosing unit is configured to: 
 diagnose that the selective catalytic reduction NOx catalyst is in a normal state when a NOx purification rate calculated based on an output value of the sensor is equal to or higher than a threshold; and 
 diagnose that the selective catalytic reduction NOx catalyst is in an abnormal state when the NOx purification rate calculated based on the output value of the sensor is lower than the threshold, and 
 the diagnosis control unit is configured to correct the threshold or the NOx purification rate based on the amount of decline in output of the sensor as calculated by the calculating unit. 
 
     
     
       4. The abnormality diagnosis apparatus for the exhaust gas purification apparatus according to  claim 3 , wherein the diagnosis control unit is configured to correct the threshold by adding an amount of rise of a NOx purification rate calculated based on the amount of decline in output to the threshold. 
     
     
       5. The abnormality diagnosis apparatus for the exhaust gas purification apparatus according to  claim 3 , wherein the diagnosis control unit is configured to correct a NOx purification rate calculated based on an output value of the sensor by subtracting an amount of rise of a NOx purification rate calculated based on the amount of decline in output from the NOx purification rate. 
     
     
       6. The abnormality diagnosis apparatus for the exhaust gas purification apparatus according to  claim 4 , wherein
 the NOx concentration estimating unit is configured to estimate a NOx concentration in exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state, 
 the ammonia concentration estimating unit is configured to estimate an ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state, 
 the calculating unit is configured to calculate the amount of decline in output when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state based on the NOx concentration and the ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state, and 
 the diagnosis control unit is configured to correct the threshold by adding, to the threshold, an amount of rise of a NOx purification rate calculated based on the amount of decline in output when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state. 
 
     
     
       7. The abnormality diagnosis apparatus for the exhaust gas purification apparatus according to  claim 4 , wherein
 the NOx concentration estimating unit is configured to estimate a NOx concentration in exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in a normal state, and a NOx concentration in exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state, 
 the ammonia concentration estimating unit is configured to estimate an ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in a normal state, and an ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state, 
 the calculating unit is configured to calculate the amount of decline in output when assuming that the selective catalytic reduction NOx catalyst is in a normal state based on the NOx concentration and the ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in a normal state and, at the same time, calculate the amount of decline in output when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state based on the NOx concentration and the ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state, and 
 the diagnosis control unit is configured to correct the threshold by adding, to the threshold, a value which is equal to or larger than an amount of rise of a NOx purification rate calculated based on the amount of decline in output when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state and, at the same time, which is equal to or smaller than a sum value of the amount of rise of a NOx purification rate calculated based on the amount of decline in output when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state and the amount of rise of a NOx purification rate calculated based on the amount of decline in output when assuming that the selective catalytic reduction NOx catalyst is in a normal state. 
 
     
     
       8. The abnormality diagnosis apparatus for the exhaust gas purification apparatus according to  claim 1 , wherein
 the diagnosis control unit is configured to: 
 make the diagnosing unit perform an abnormality diagnosis of the selective catalytic reduction NOx catalyst when an amount of decline in output of the sensor as calculated by the calculating unit is equal to or smaller than a prescribed amount; and 
 make the diagnosing unit not perform an abnormality diagnosis of the selective catalytic reduction NOx catalyst when the amount of decline in output of the sensor as calculated by the calculating unit is larger than the prescribed amount. 
 
     
     
       9. The abnormality diagnosis apparatus for the exhaust gas purification apparatus according to  claim 1 , wherein
 the diagnosing unit is configured to: 
 diagnose that the selective catalytic reduction NOx catalyst is in a normal state when an output value of the sensor is equal to or smaller than a threshold; and 
 diagnose that the selective catalytic reduction NOx catalyst is in an abnormal state when the output value of the sensor is larger than the threshold, and 
 the diagnosis control unit is configured to correct the threshold or the output value of the sensor based on the amount of decline in output of the sensor as calculated by the calculating unit. 
 
     
     
       10. The abnormality diagnosis apparatus for the exhaust gas purification apparatus according to  claim 9 , wherein the diagnosis control unit is configured to correct the threshold by subtracting the amount of decline in output from the threshold. 
     
     
       11. The abnormality diagnosis apparatus for the exhaust gas purification apparatus according to  claim 9 , wherein the diagnosis control unit is configured to correct the output value of the sensor by adding the amount of decline in output to the output value of the sensor. 
     
     
       12. The abnormality diagnosis apparatus for the exhaust gas purification apparatus according to  claim 10 , wherein
 the NOx concentration estimating unit is configured to estimate a NOx concentration in exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state, 
 the ammonia concentration estimating unit is configured to estimate an ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state, 
 the calculating unit is configured to calculate the amount of decline in output when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state based on the NOx concentration and the ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state, and 
 the diagnosis control unit is configured to correct the threshold by subtracting the amount of decline in output when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state from the threshold. 
 
     
     
       13. The abnormality diagnosis apparatus for the exhaust gas purification apparatus according to  claim 10 , wherein
 the NOx concentration estimating unit is configured to estimate a NOx concentration in exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in a normal state, and a NOx concentration in exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state, 
 the ammonia concentration estimating unit is configured to estimate an ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in a normal state, and an ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state, 
 the calculating unit is configured to calculate the amount of decline in output when assuming that the selective catalytic reduction NOx catalyst is in a normal state based on the NOx concentration and the ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in a normal state and, at the same time, calculate the amount of decline in output when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state based on the NOx concentration and the ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state, and 
 the diagnosis control unit is configured to correct the threshold by subtracting, from the threshold, a value which is equal to or larger than the amount of decline in output when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state and, at the same time, which is equal to or smaller than a sum value of the amount of decline in output when assuming that the selective catalytic reduction NOx catalyst is in an abnormal state and the amount of decline in output when assuming that the selective catalytic reduction NOx catalyst is in a normal state. 
 
     
     
       14. The abnormality diagnosis apparatus for the exhaust gas purification apparatus according to  claim 1 , wherein the diagnosing unit is configured to perform an abnormality diagnosis when ammonia or an ammonia precursor is being supplied from the supplying unit. 
     
     
       15. The abnormality diagnosis apparatus for the exhaust gas purification apparatus according to  claim 14 , wherein a supply amount of ammonia or an ammonia precursor by the supplying unit is increased when an abnormality diagnosis is performed by the diagnosing unit as compared to when an abnormality diagnosis is not performed by the diagnosing unit.